Project Summary Organisms evolved to survive and reproduce in an environment that changes along many dimensions. As a consequence, genomes respond to environmental stress through major alterations in gene expression. In eukaryotes, environmental and developmental signals interact with the genome through chromatin, thus understanding the role of chromatin structure in the activation of stress dependent gene expression programs is crucial. Therefore, we need new studies dissecting the mechanisms controlling stress responses that are well grounded in organismal physiology. Plants provide a unique opportunity to address this issue. To support a sessile lifestyle, plants evolved sophisticated mechanisms to adjust their growth and physiology to confront environmental challenges such as drought, a major limitation on plant growth. Importantly, both the frequency and severity of droughts will likely increase in the near future due to climate change. In dry soil plant cells experience osmotic stress which triggers the differential expression of thousands of genes, a reprogramming of gene expression known as the osmotic stress response. Stressed plant tissues accumulate the hormone abscisic acid (ABA), and ABA signaling further coordinates drought stress transcriptional responses. Despite the massive scale of these transcriptional changes we know little about their accompanying regulation by chromatin structure nor is it clear how ABA hormone signaling is integrated into the larger osmotic stress response. Additionally, our knowledge of the transcriptional regulators mediating the osmotic stress response is far from complete. Using the reference plant Arabidopsis thaliana, a powerful genetic model system, the proposed research will uncover the regulatory program mediating the response to osmotic stress in plant roots. The specific aims of the proposal are: (1) To test if osmotic stress driven changes in chromatin structure prime the genome for subsequent stress hormone induced gene expression using cell type specific genomics. (2) To carry out a focused RNAi screen to identify novel transcriptional regulators functioning in the osmotic stress response. (3) To directly visualize the impact of osmotic stress on nuclear structure and dynamics using confocal microscopy on live plant roots.